550 miles out of a tank of diesel

our car (2002 2.2 dci The Race, 103.000km) regularly passes 1000km (620 miles) on a tank, - by which time we pour some 75-80 litres into it.

Provided our tanks are equal size (which they of course should be) 550miles seems a bit short - something close to 33 mpg. We get 36-37 as the norm. But then again, thats within 12% so maybe its just topology and driving style ?

I have a good measurement of 300 km strictly 125 km/h (GPS) steady (sort of boring) motorway-driving with a constant speed - where our car apparently did just over 15 km/l (42 mpg) - which probably is as good as it gets. This is derived from the notoriously inaccurate tripcomputer, corrected with the knowledge of how much diesel I pour into the tank etc.

Driving 90 km/h will probably make it slightly better, but changing speed is the costly part. Seen from a fuel-economy viewpoint, it just doesn't like queues/traffic.

Also, how long can I go on reserve

dunno, - and since its a diesel, I haven't been so brave to actually find out - so we fill it up either before or just after it reminds us to.

I think it is normally 5 liters, i.e. 40 miles or so?